Due to strong multipath, there are limitations in implementing terrestrial digital transmission system requiring a high data rate. Also, it is difficult to transmit data at a high speed due to time dispersion of a channel. As a data rate becomes higher, inter-symbol interference (ISI) increases and a signal distortion and signal-to-noise ratio (SNR) of a channel are degraded, causing limitation in the amount of information to be transmitted over a transmission bandwidth of the channel. To overcome these limitations, an OFDM scheme was proposed. The OFDM scheme is a multi carrier modulation scheme that transmits data using a large number of subcarriers. Specifically, European Digital Video Broadcasting-Terrestrial (DVB-T) standard and Korean Terrestrial Digital Multimedia Broadcasting (T-DMB) standard adopted the OFDM modulation scheme and are used in digital audio broadcasting, satellite broadcasting, high-speed wireless LAN, mobile communications, and so on.
A T-DMB system is based on a Eureka-147 Digital Audio Broadcasting (DAB) system and utilizes an OFDM transmission technique. The T-DMB system is used for smoothly receiving broadcasting signals in poor environments, e.g., downtown surrounded by high buildings, express highway, etc.
A T-DMB receiver utilizes a differential detection technique for demodulation. A coherent detection technique was recently proposed for improving reception performance. Channel equalization for coherent detection is performed in a frequency domain using a one-tap equalizer. A tap coefficient of the equalizer can be obtained by estimating a reception channel and calculating a reciprocal of the estimated channel coefficient. The reciprocal of the estimated channel coefficient may become very large in a deep fading channel even though the channel estimation is accurately performed. This may cause noise amplification in a channel equalization procedure.
Noise component remaining after the channel estimation serves as a factor that degrades a reception performance of the T-DMB receiver. The T-DMB receiver performs the channel estimation using a known symbol, called a pilot. The channel estimation is achieved by a pilot signal extraction and channel coefficient estimation. The channel estimation performance is determined by noise component remaining in the channel coefficient estimated in association with SNR. Thus, it is very important to reduce the remaining noise component.
A baseband configuration of a T-DMB system will be described below with reference to FIG. 1. An OFDM transmitter groups and maps binary bits in accordance with a modulation scheme. After pilot insertion, the modulated data is transmitted to an Inverse Fast Fourier Transform (IFFT) block and then converted into a time-domain signal. By copying a portion of the end of the OFDM symbol, a guard interval is inserted for eliminating inter-symbol interference (ISI).
The generated OFDM signal is transmitted over a frequency selective fading channel with Additive White Gaussian Noise (AWGN). An OFDM receiver removes the guard interval from a received signal and performs an FFT operation to obtain a frequency-domain signal.
In the OFDM-based communication system, the transmitter inserts a pilot known to the receiver in order for channel estimation, and the receiver estimates a channel using the pilot inserted by the transmitter and compensates for channel distortion. As illustrated in FIG. 2, the inserted pilot signal may be classified into a block-type pilot pattern 2A and a comb-type pilot pattern 2B according to pilot arrangement. The T-DMB system utilizes the block-type pilot pattern 2A. As illustrated in FIG. 3, a pilot block is inserted at the same positions in every 76 OFDM symbols.
Referring to FIG. 2, since the pilot symbols in the block-type pilot pattern 2A are inserted into the entire OFDM symbols at regular time intervals, the block-type pilot pattern 2A is suitable for slow-fading channel estimation. The block-type pilot pattern 2A is used in the T-DMB system. The channel estimation is carried out by applying a Least Squares (LS) or Minimum Mean Square Error (MMSE) method to a corresponding frequency-domain channel coefficient in the pilot-inserted OFDM symbols.
Since the pilot symbols in the comb-type pilot pattern 2B are inserted into all OFDM symbols, the comb-type pilot pattern 2B is suitable for fast-fading channel estimation. The comb-type pilot pattern 2B is used in the DVB-T system. However, since the pilots in the comb-type pilot pattern 2B are inserted into one OFDM symbol at regular frequency intervals, the channel estimation using the pilots is impossible in a data interval between the pilots.
In order to estimate the channel coefficient in the data interval, an interpolation method is carried out to estimate a coefficient value corresponding to a subcarrier frequency between adjacent pilots using the channel coefficient value estimated through an LS or MMSE method. Thus, the channel estimation performance of the OFDM system with the comb-type pilot pattern 2B is different according to the interpolation method. Linear interpolation, Lagrange interpolation, cubic interpolation, Spline interpolation, and Gaussian interpolation are generally used.
A conventional LS estimation method, which is most widely used for the channel estimation in the OFDM system, will be described below with reference to FIGS. 4 to 7.
FIG. 4 illustrates an LS channel estimation according to a pilot insertion pattern, and
FIG. 5 illustrates a block diagram of a conventional channel estimation apparatus in a block-type pilot pattern. FIG. 6 illustrates a block diagram of a conventional channel estimation apparatus in a comb-type pilot pattern, and FIG. 7 illustrates a conventional interpolation channel estimation in a comb-type pilot pattern.
Referring to FIG. 5, which shows the channel estimation in the block-type pilot pattern 2A, a pilot extractor 51 extracts a pilot signal YP from a received signal Y, a pilot generator 52 generates a known pilot signal XP, and a pilot estimator 53 estimates a channel coefficient
Ĥ
using the extracted pilot signal YP and the generated pilot signal XP, based on an LS method expressed as Eq. 1 below.
MathFigure 1
                              H          ^                =                                            [                                                                    H                    ^                                    ⁡                                      (                    0                    )                                                  ⁢                                                      H                    ^                                    ⁡                                      (                    1                    )                                                  ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                                                      H                    ^                                    (                                      N                    -                    1                                    ]                                            )                        T                    ⁢                                          ⁢                                          =                                    [                                                                                          Y                      P                                        ⁡                                          (                      0                      )                                                                                                  X                      P                                        ⁡                                          (                      0                      )                                                                      ⁢                                                                            Y                      P                                        ⁡                                          (                      1                      )                                                                                                  X                      P                                        ⁡                                          (                      1                      )                                                                      ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                                                                            Y                      P                                        ⁡                                          (                                              N                        -                        1                                            )                                                                                                  X                      P                                        ⁡                                          (                                              N                        -                        1                                            )                                                                                  ]                        T                                              [                  Math          .                                          ⁢          1                ]            
whereĤ(=[Ĥ(0)Ĥ(1) . . . Ĥ(N−1)]T)
is the estimated channel;XP(=[XP(0)XP(1) . . . XP(N−1)]T)
is the pilot signal known to the transmitter and the receiver;YP(=[YP(0)YP(1) . . . YP(N−1)]T 
is the pilot signal extracted from the received signal;
N represents the number of subcarriers of the OFDM symbol; and
T represents a transpose.
Referring to FIGS. 6 and 7, which show the channel estimation in the comb-type pilot pattern 2B, a pilot extractor 61 extracts a pilot signal YP from a received signal Y, a pilot generator 62 generates a known pilot signal XP, and a pilot estimator 63 estimates a channel coefficient
ĤP 
using the extracted pilot signal YP and the generated pilot signal XP, based on an LS method expressed as Eq. 2 below. An interpolator 64 estimates a channel coefficient Ĥ
of a received channel containing a channel coefficient of a subcarrier on which data is loaded.
MathFigure 2
                                                                                          H                  ^                                P                            =                                                                                          H                      ^                                        P                                    ⁡                                      (                    0                    )                                                  ⁢                                                                            H                      ^                                        P                                    ⁡                                      (                    1                    )                                                  ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                                                                            H                      ^                                        P                                    (                                                            N                      P                                        -                    1                                    ]                                                      )                    T                ⁢                                  ⁢                                  =                              [                                                                                Y                    P                                    ⁡                                      (                    0                    )                                                                                        X                    P                                    ⁡                                      (                    0                    )                                                              ⁢                                                                    Y                    P                                    ⁡                                      (                    1                    )                                                                                        X                    P                                    ⁡                                      (                    1                    )                                                              ⁢                                                          ⁢              …              ⁢                                                          ⁢                                                                    Y                    P                                    ⁡                                      (                                                                  N                        P                                            -                      1                                        )                                                                                        X                    P                                    ⁡                                      (                                                                  N                        P                                            -                      1                                        )                                                                        ]                    T                                    [                  Math          .                                          ⁢          2                ]            
whereĤP(=[ĤP(0)ĤP(1) . . . {circumflex over (H)}P(NP−1)]T)
is the estimated channel;XP(=[XP(0)XP(1) . . . XP(NP−1)]T)
is the pilot signal known to the transmitter and the receiver;YP(=[YP(0)YP(1) . . . YP(NP−1)]T 
is the pilot signal extracted from the received signal;
NP represents the number of subcarriers, on which data is loaded, among N subcarriers of the OFDM symbol; and
T represents a transpose.
Generally, the OFDM system utilizes the one-tap equalizer in the frequency domain in order for channel equalization, and the tap coefficient of the equalizer is obtained using the reciprocal of the frequency domain of the estimated channel coefficient Ĥ. Thus, the channel equalization performance is directly influenced by the channel estimation performance.
However, if a deep fading exists in the received channel, the coefficient value for channel equalization becomes very large even though the channel estimation is accurately performed. This may cause noise amplification in the channel equalization procedure. Further, noise component always remains in the estimated channel, and such a noise component may degrade the performance of the equalizer.
Consequently, there is a need for channel estimation apparatus and method that can prevent noise amplification phenomenon caused by the deep fading channel and reduce noise component remaining in the estimated channel.